def binary_search(l, item):
    low = 0
    high = len(l) - 1
    while low <= high:
        mid = int((low + high) / 2)
        guess = l[mid]
        if guess == item:
            return mid
        if guess > item:
            high = mid - 1
        else:
            low = mid + 1
    return None

my_list = [1, 3, 5, 7, 9]
print(binary_search(my_list,5))
print(binary_search(my_list,-1))